1. Field of the Invention
The present invention relates to a mechanical pencil with a chuck which permits a lead to advance but prevents the lead from retracting, and more particularly to a mechanical pencil with a chuck made of plastics.
2. Description of the Prior Art
One such mechanical pencil with a chuck U.S. Pat. No. 4,627,756, discloses a known mechanical pencil having a chuck whose mechanism is described in detail, but whose material is not described. In general, the chuck is made of metal or plastics. The chuck made of plastics, e.g. synthetic resin, has the advantage of being less expensive to manufacture, compared with a metal chuck which is generally manufactured from a shaft or pipe made of metal such as brass, and the plastic chuck does not damage it is grasping, nor destroy it when an excessive writing force is applied thereto.
The known plastic chuck is formed like a tubular member axially divided into two pieces from its tip end portion toward its rear end portion, the divided pieces flaring away from each other radially. The tip end portions of the divided pieces are for grasping a lead with their inner surfaces, are engaged at their outer surfaces with a chuck ring which is axially slidably positioned around the tubular member. The tip end portions are forced by the chuck ring toward the center or the axis of the tubular member, so that the lead is grasped and held therein.
In a mechanical pencil with the above-described plastic chuck, however, the lead grasping force is sometimes insufficient to hold the lead, so that the lead may be retracted into a tip member or a lead guide tube of the mechanical pencil when a large writing force is applied to the lead.
Further, in the known plastic chuck having two divided pieces, the tip end portions are formed into a cylindrical tube of an oval cross-section when the tubular member is engaged with the chuck ring, so as to focus the lead grasping force on the axis of the lead. In that chuck, a contacting area between the outer surface of the chuck and the inner surface of the chuck ring to be mounted thereon is small due to a difference in a radius of curvature therebetween. Accordingly, the outer surface of the tip end portion of the chuck is likely to be partially worn and consequently formed thereon is a stepped portion which prevents the chuck ring from moving toward the tip end portion, so that the lead grasping force is reduced and a durability of the chuck is affected adversely.